Iron-type golf club and FRP shaft therefor

ABSTRACT

A club shaft for an iron-type golf club is made of a fiber reinforced resin. The weight W of the club shaft per 39 inches is not less than 30 grams and not more than 60 grams. The average bending rigidity EIa of the club shaft is not more than 5.0 kgf sq.m. An expression (1) EIa&gt;=0.05 W+2.0 is satisfied. Preferably, an expression (2) EIa&gt;=0.05 W+2.5 is satisfied. More preferably, an expression (3) EIa&gt;=(1/15)W+2.0 is satisfied. Even if golfers can not make powerful swings through the swing speed is high, the carry distance and the directionality of the hit ball can be improved.

BACKGROUND OF THE INVENTION

The present invention relates to an iron-type golf club, more particularly to a FRP shaft having a specific relation between the bending rigidity and weight of the shaft.

An optimum flexure of the golf club shaft during golf swing can accelerate the head speed just before impact and increase the dynamic loft angle at the impact. Thus, the launch angle of the ball can be optimized.

In the case of golfers with powerful right swings such as professional golfers and advanced golfers, usually, the head speed is high and the flexure during golf swing is large. Therefore, club shafts with relatively heavy weight and less flexibility (stiff) are conventionally used in iron-type golf clubs targeted at such users on the other hand, in the case of golfers with less powerful swings such as beginners and senior golfers, usually, the head speed is relatively low and the flexure is small. Therefore, club shafts with light weight and high flexibility are conventionally used in iron-type golf clubs targeted at such users. Thus, the club shafts of the conventional iron-type golf clubs are generally categorized into the above-explained two groups.

In recent years, on the on other hand, FRP club shafts made of fiber reinforced resins are widely used because the weight, bending rigidity and the like can be controlled easily in comparison with metal club shafts.

In the case that a golfer is not powerful but adept at the body turn or body spin on golf swing and thus the swing speed is high, if the golfer uses the former type of a club shaft with relatively heavy weight and less flexibility, then there is a tendency that the golf club can not come back to its right position at impact, thus the club face can not be squared and as a result the directions of the hit balls become unstable. If on other hand the golfer uses the later type of a club shaft with light weight and high flexibility, then there is a tendency that the club shaft is bent too much during golf swing, and as a result the club face can not be squared at impact and the directions of the hit balls become unstable.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide an iron-type golf club and a club shaft therefor, which is suitable for beginners and senior golfers, and in which the directionality of the hit ball become stable, and at the same time, the golf swing power can be transferred to the club head at the maximum, without changing the current golf swing tempo.

According to the present invention, a club shaft for an iron-type golf club is made of a fiber reinforced resin, and has a per-39 inch weight W of not less than 30 grams and not more than 60 grams, and an average bending rigidity EIa of not more than 5.0 kgf sq.m, wherein an expression (1) EIa>=0.05 W+2.0 is satisfied. Preferably, an expression (2) EIa>=0.05 W+2.5 is satisfied. More preferably, an expression (3) EIa>=(1/15)W+2.0 is satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an iron-type golf club according to the present invention.

FIG. 2 is a diagram for explaining a method for measuring the bending rigidity of the club shaft.

FIG. 3 is a graph showing the ranges defined by the expressions (1), (2) and (3).

FIG. 4 shows a set of prepreg pieces which can be used to make the club shaft.

FIG. 5 shows prepreg pieces of the club shafts used in the undermentioned comparison tests.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail in conjunction with accompanying drawings.

According to the present invention, golf club 1 is an iron-type golf club such as first-ninth irons, sand-wedge, pitching-wedge, approach-wedge and lob-wedge.

The golf club 1 comprise a club shaft 2, a club head 3 attached to the tip end 2 a, and a grip 4 attached to the butt end 2 b.

The club head 3 is, in its principal part(s), made of at least one kind of a metal material, e.g. stainless steels, aluminum alloys, titanium alloys, pure titanium and the like. It is of course possible to use a nonmetal material, e.g. fiber reinforced resins and the like in order to form a part of the club head 3. The club head 1 can be either a hollow structure or a solid structure. Preferably, the club head 1 has a volume in a range of from 25 to 40 cc, and a weight in the range of from 220 to 300 grams.

The grip 4 can be a rubber grip, resin grip, leather grip or the like.

The club shaft 2 is a circular tube made of a fiber reinforced resin and tapered from a butt end 2 b towards a tip end 2 a.

As to the resin, thermosetting resins and thermoplastic resins can be used. As the thermosetting resin, epoxide resin, unsaturated polyester resin, phenol resin, melamine resin, urea resin, diallyl phthalate resin, polyurethane resin, polyimide resin, silicon resin and the like may be used. As the thermoplastic resin, polyamide resin, saturated polyester resin, polycarbonate resin, polystyrene resin, polyethylene resin, polyvinyl acetate resin, AS resin, methacrylic resin, polypropylene resin, fluorocarbon resin and the like may be used.

As to the fiber reinforcement of the fiber reinforced resin, inorganic fibers (carbon fibers, glass fibers, boron fibers, silicon carbide fibers, alumina fibers and the like), organic fibers (polyethylene fibers, polyamide fibers and the like), and metal fibers can be used alone or in combination. Especially, those having a tensile elastic modulus of from 3 to 90 tonF/sq.mm are preferred in view of the strength and weight reduction.

The club shaft 2 has a length SL inch and a weight Wr grams, and the value W (=Wr×39/SL) is preferably set in a range of not less than 30, preferably not less than 35, but not more than 60, preferably not more than 55.

This value W expresses a weight of the club shaft converted into a 39-inch long shaft (hereinafter the “per-39 inch weight” W).

Usually, the length of the club shaft is changed according to specifications. e.g. types of club head, loft angle, user's preference and the like. Accordingly, it will be not significant to define the bending rigidity by referring to such unspecified length. In this invention, therefore, based on a new criterion, the weight of the club shaft converted into a constant length of 39 inches, the weight and bending rigidity of the club shaft are defied.

If the per-39 inch weight W is less than 30 grams, then in comparison with conventional club shafts, as the club shaft becomes very light, there is a possibility that the user feels odd at the time of address. As a result, the golf swing and the directionality of the hit ball might become unstable. Further, there is a possibility that the rigidity and strength of the club shaft 2 becomes insufficient. If the per-39 inch weight W is more than 60 grams, then it becomes difficult for the target users to increase the swing speed and the carry distance tends to decrease.

For the similar reasons, the actual weight Wr is preferably set in a range of not less than 25 grams, more preferably not less than 30 grams, still more preferably not less than 35 grams, but not more than 65 grams, more preferably not more than 60 grams, still more preferably not more than 55 grams.

If the actual club shaft length SL is too short, the length of the club shaft can not be utilized to increase the head speed. If the actual club shaft length SL is too long, it is difficult to swing the golf club and as a result, there is a possibility that the head speed is decreased.

Therefore, the actual club shaft length SL is preferably set in a range of not less than 500 mm, more preferably not less than 525 mm, still more preferably not less than 550 mm, but not more than 1100 mm, more preferably not more than 1075 mm, still more preferably not more than 1050 mm.

The club shaft 2 has an average bending rigidity EIa in a range of not more than 5.0 kgf sq.m, preferably not more than 4.5 kgf sq.m, more preferably not more than 4.0 kgf sq.m,

The average bending rigidity EIa is the mean value of the bending rigidity EI measured at a plurality of positions 2P which are, as shown in FIG. 2, set up along the club shaft 2 at a distance of 130 mm. from the tip end 2 a and at intervals of 100 mm from this 130 mm position toward the butt end 2 b. Accordingly, the measuring position 2P most close to the tip end 2 a is at 130 mm therefrom. The distance between the butt end 2 b and the measuring position 2P most close to the butt end 2 b depends on the shaft length SL but it should be not less than 130 mm but less than 230 mm.

The bending rigidity EI at each of the positions 2P is measured with a universal material testing machine through a three point bending test. More specifically, as shown in FIG. 2, the club shaft 2 is supported by two fulcrums J1 and J2 spaced apart from each other by 200 mm, so that the central axis CL of the shaft is kept horizontally, and one of the above-mentioned measuring positions 2P is positioned at the midpoint of the two fulcrums J1 and J2. Then, an indenter P is let down onto the measure position 2P from right above at a descending speed of 5 mm/minutes when the load applied to the measure position 2P is increased and reaches to 20 kgf, the indenter P is stopped. And the amount of deflection of the club shaft 2 is measured at the position 2P.

From the amount of deflection, the bending rigidity EI at the position 2P is computed, using the following expression:

Bending rigidity EI=load×(distance between J1 and J2)̂3/(48× amount of deflection)

wherein the tips of the fulcrums J1 and J2 are rounded by a radius of 12.5 mm, the tip of the indenter is rounded by a radius 6.00 mm, the units of the distance and the amount of deflection are “meter”, and the unit of the load (force) is “kgf”.

If the average bending rigidity EIa exceeds 5.0 kgf sq.m, it become difficult to appropriately bend the club shaft 2 during golf swing, and the head speed and dynamic loft angle at impact can not be increased in order to increase the carry distance. If the average bending rigidity EIa is too small, since the club shaft is bent too much during golf swing, the swing is difficult, and the directionality of the hit ball becomes unstable. Further, it is difficult to make powerful shots.

In consideration of these facts, the per-39 inch weight W and average bending rigidity EIa are configured to satisfy the following expression (1)

EIa>=0.05 W+2.0,   (1)

preferably to satisfy the following expression (2)

EIa>=0.05 W+2.5,   (2)

more preferably to satisfy the following expression (3)

EIa>=(1/15)W+2.0.   (3)

It is also preferable that the following expression (4) is satisfied:

EIa=<3/20 W.   (4)

FIG. 3 is a graph showing the ranges of the average bending rigidity EIa and per-39 inch weight W defined by the expressions (1)-(4). In the graph, an ellipse indicates a range in which conventional club shafts are mapped such a design concept that when the per-39 inch weight W is small, the average bending rigidity EI is also small, can be read into this distribution map. As having been explained, if a club shaft with such light weight and low rigidity is used by a golfer with less powerful swings but high head speed, the club shaft is excessively flexed during golf swing, and there is a tendency that the club face at impact becomes unstable, and accordingly, the directionality becomes worse. In the present invention, as the per-39 inch weight W is limited in a specific range, the club shaft 2 is provided with an optimum weight by which it becomes easy to address and swing. Further, as the lower limit for the average bending rigidity EIa is defined within a specific range, a suitable flexure can be secured during golf swing.

Furthermore, as the expression (1) is satisfied, the average bending rigidity EIa becomes relatively large for the club shaft weight in comparison with the club shafts of the ordinary concept, therefore, the golfer with less powerful swings but high head speed, can obtain a perfect flexure, without the need to changing the previous golf swing timing, and as a result, the directionality and carry distance of the hit ball can be greatly improved.

It is preferable that, in addition to the above limitation of the average bending rigidity EIa, the bending rigidity EI at each measuring position 2P is limited as follows.

The bending rigidity EI can be expressed as a function EI(x) of a distance “x” in millimeter of the measure position from the tip end 2 a of the club shaft 2, namely, El(130), El(230), El(330), El(430), El(530), El(630), El(730), El(830), El(930), El(1030), El(1130) . . . .

As explained above, the preferable range of the shaft length SL is 500 to 1100 mm.

If the shaft length SL is 1100 mm, the distance “x” is 130, 230, 330, 430, 530, 630, 730, 830 and 930 mm. If the shaft length SL is 500 mm, the distance “x” is 130, 230 and 330 mm. Thus, in the case that SL is 500 to 1100 mm, the number (m) of the measuring points 2P becomes 3 to 9 depending on the shaft length SL.

The bending rigidity EI(130), EI(230) and EI(330) at the first, second and third measuring points from the tip end 2 a toward the butt end 2 b are preferably set in a range of not less than 1.0 kgf sq.m, more preferably not less than 1.2 kgf sq.m, still more preferably not less than 1.4 kgf sq.m, but not more than 4.0 kgf sq.m, more preferably not more than 3.8 kgf sq.m, still more preferably not more than 3.5 kgf sq.m.

If at least one of EI(130), EI(230) and EI(330) is less than 1.0 kgf sq.m, the flexure of the tip end part of the club shaft becomes excessive, and the durability is decreased. Further, the directionality of the hit ball is liable to become unstable. If at least one of EI(130), EI(230) and EI(330) is more than 4.0 kgf sq.m, the flexure of the tip end part of the club shaft becomes insufficient, and it is difficult to accelerate the head speed just before impact. Moreover, the hit feeling is liable to become worse because the shock of hitting a ball transmitted to the user's hands increases when the number (m) of the measuring points 2P is 7 or more, the bending rigidity is expressed as follows: EI(130), EI(230), EI(330), EI(n*100+30), EI{(m−2)*100+30)}, EI{(m−1)*100+30)}, and EI(m*100+30), wherein “n” is an integer variable more than 3 and less than m−2. (when m=7, n=4), (when m=9, n=4, 5 and 6)

If the bending rigidity EI(n*100+30) of the middle part of club shaft 2 is less than 1.5 kgf sq.m, since the flexure of the shaft during golf swing increases in the middle part, there is a tendency that the user upsets the golf swing timing or golf swing tempo. If the bending rigidity EI(n*100+30) is more than 6.5 kgf sq.m, as the flexure is decreased, there is a possibility that the acceleration of the head speed becomes insufficient, and the power of golf swing can not be transferred to the club head effectively.

Therefore, the bending rigidity EI(n*100+30) is preferably set in a range of not less than 1.5 kgf sq.m, more preferably not less than 1.7 kgf sq.m, still more preferably not less than 1.9 kgf sq.m, but not more than 6.5 kgf sq.m, more preferably not more than 6.0 kgf sq.m, still more preferably not more than 5.5 kgf sq.m.

Further, the bending rigidity EI(n*100+30) is more than EI(130), EI(230) and EI(330).

when the integer variable “n” include a plurality of integers, it is preferred that the bending rigidity EI(n*100+30) gradually increases towards the butt end 2 b.

The bending rigidity EI{(m−2)*100+30)}, EI{(m−1)*100+30)} and EI(m*100+30) at the last three measuring points on the butt end side are preferably set in a range of from 3.5 to 9.0 kgf sq.m.

If at least one of these is less than 3.5 kgf sq.m, since the flexure of the shaft during golf swing increases, there is a tendency that the user upsets the golf swing timing or golf swing tempo. If more than 9.0 kgf sq.m, it becomes difficult for the user to feel the flexure of the club shaft 2 during golf swing, and thus difficult to swing the golf club.

Further, it is preferred that the bending rigidity gradually increases towards the butt end 2 b, namely, EI{(m−2)*100+30)}<EI{(m−1)*100+30)}<EI(m*100+30), and EI{(m−2)*100+30)} is 3.5 to 6.5 kgf sq.m, EI{(m−1)*100+30)} is 3.8 to 7.0 kgf sq.m, EI(m*100+30) is 4.0 to 9.0 kgf sq.m.

The club shaft 2 is, as shown in FIG. 4, made from pieces S1 to S2 of prepreg (hereinafter, generically, the “prepreg pieces S”).

The prepreg pieces s are wound around a mandrel (not shown) into a laminated tube. The mandrel is removed from the tube. An inflatable bladder (not shown) is inserted into the laminated tube. The laminated tube is put in a mold (not shown) together with the bladder. By inflating the bladder and applying heat, the laminated tube is cured and molded into the FRP shaft 2.

FIG. 4 shows an example set of prepreg pieces S constituting a shaft 2, wherein the prepreg pieces S are wound in the order from the prepreg piece shown on the top of the figure to that on the bottom.

The prepreg pieces S include: small tip-end-side prepreg pieces S1 forming a part near the tip end 2 a of the club shaft 2; and long prepreg pieces S2 continuous over the entire shaft length.

Aside from this example, various combinations of prepreg pieces are possible. For example, a small butt-end-side prepreg piece forming a part near the butt end 2 b of the club shaft 2 can be used.

In order to control the rigidity and to increase the strength of the tip end part of the club shaft 2, the number of layers formed by the winded prepreg pieces S1 is preferably at least 2, but at most 20, preferably not more than 19, more preferably not more than 18.

The orientation angle of the fiber reinforcements (f) in the tip-end-side prepreg piece S1 can be set in a wide range of from 0 to 90 degrees with respect to the direction of the shaft axis. For example, if it is desired to increase the bending rigidity EI near the tip end 2 a, it is preferred to set the orientation angle in a range of not more than 10 degrees, most preferably at 0 degree. If it is desired to increase the torsional rigidity, it is preferred to set the orientation angle in a range of from 40 to 50 degrees, most preferably at 45 degrees.

The shape of the tip-end-side prepreg piece S1 (the shape developed on a plane) can be either a quadrangular shape S1 a or a triangular shape S1 b.

In either case, it is preferable that a side on the butt end side (and at least one of two lateral sides in the case of the quadrangular shape S1 a) is inclined at an angle of 30 to 60 degrees with respect to the longitudinal direction of the shaft in order to avoid a formation of a large step difference in rigidity.

The long prepreg pieces S2 determine the fundamental bending rigidity and strength of the club shaft 2.

If the number of layers formed by the winded long prepreg pieces S2 is less than 5, there is a possibility that the club shaft 2 lacks necessary rigidity and strength. If more than 20, not only the production efficiency is lowered, but also the likelihood of getting voids between the layers is increased. Therefore, the number of layers formed by the winded long prepreg pieces S2 is set in a range of not less than 5, preferably not less than 6, more preferably not less than 7, but not more than 20, preferably not more than 19, more preferably not more than 18.

The long prepreg pieces S2 include:

a bias piece S2 a whose fiber reinforcements (f) are oriented at an angle of not less than 10 degrees, preferably not less than 20 degrees, but not more than 80 degrees, preferably not more than 70 degrees; a parallel piece S2 b whose fiber reinforcements (f) are oriented at an angle of substantially 0 degree; and optionally an orthogonal piece S2 c whose fiber reinforcements (f) are oriented at an angle of substantially 90 degrees, wherein the angles are referred to with respect to the direction of the shaft axis.

The bias piece S2 a is most useful for increasing the torsional rigidity. For that purpose, the number of layers formed by the winded bias piece(s) S2 a is set in a range of not less than 2, preferably not less than 3, more preferably not less than 4, but not more than 12, preferably not more than 11, more preferably not more than 10.

It is preferable that the layers of the bias piece(s) S2 a include at least one pair of layers whose orientation angles are directionally opposite with respect to the longitudinal direction of the shaft, more preferably orientation angles are same in the absolute values (for example 45 degrees with respect to the longitudinal direction).

The parallel piece S2 b is most useful for increasing the bending rigidity. For that purpose, the number of layers formed by the winded parallel piece(s) S2 b is set in a range of not less than 2, preferably not less than 3, but not more than 10, preferably not more than 9, more preferably not more than 8.

The use of the orthogonal piece S2 c in combination with the bias piece S2 a and parallel piece S2 b is very useful for increasing the crush strength of the shaft.

If the club shaft is already provided with a sufficient crush strength by the arrangement of the bias piece S2 a and parallel piece S2 b, it is not always necessary to use the orthogonal piece S2 c. It can be omitted. Even when used, the number of layers formed by the winded orthogonal piece(s) S2 c is preferably set in a range of not more than 4, more preferably not more than 3, still more preferably not more than 2, in order to avoid an unwanted increase of the club shaft weight.

Aside from the above method, various methods, e.g. so called filament winding method, sheet winding method and the like can be employed.

Comparison Tests

FRP shafts were made using prepreg pieces and comparison tests were carried out.

The prepreg pieces used are shown in FIG. 5.

The prepreg pieces A-G were winded around a mandrel from A to G. Firstly, an intermediate of the club shaft having a length of about 1200 mm was made and then cut into the target length of 1000 mm by removing both of the ends.

In order to change the average bending rigidity EIa, the tensile elastic modulus of the fiber reinforcement and the number of layers (winding number) of each prepreg piece were changed as shown in Table 1.

The specifications of the prepreg pieces are as follows:

Prepreg Tensile elastic modulus of fibers Toray “8255s-10” 30 ton/sq · mm Toray “9255s-10” 40 ton/sq · mm Toray “3255G-10” 24 ton/sq · mm NGF “E1026A-09N” 10 ton/sq · mm Toray: Toray Industries, Inc. NGF: Nippon Graphite Fiber Corporation

Golf clubs were made by attaching an iron-type golf club head and a rubber grip to the tip end and butt end, respectively, of each of the shafts. The club head was made of a stainless steel and having a loft angle of 32 degrees.

Comparison tests were carried out by ten right-handed golfers (handicap 0 to 20, age 20 to 40). Each of the golfers hit golf balls (SRI sport limited, “XXIO”) ten times par each golf club. Thus, 100 shots (10 shots×10 golfers) were made per each club.

The head speed just before impact and the ball lunching angle of each shot were measured to obtain the mean values of the 100 shots. The results are shown in Table by an index based on Ex.1 being 100.

As to the head speed, the larger index number means that the head speed was more accelerated by the flexure of the club shaft. As to the launching angle, the larger index number means that the launching angle was more increased by the optimal flexure of the club shaft.

Further, with respect to the ten shots made by each of the golfers with each club, the difference between the direction of the trajectory of the ball and the direction of the target trajectory was measured and the standard deviation was computed. With respect to each of the clubs, the mean value of the standard deviation obtained by the ten golfers was computed. The results are indicated by an index based on Ex.1 being 100,

wherein the smaller the index number, the better the directionality.

Furthermore, with respect to each of the shafts, easiness of swing was evaluated into five ranks as follows, based on the ten golfers' feeling.

5: very good, 4: good, 3: ordinary, 2: baddish, 1: bad

The mean values of the rank numbers are shown in Table 1.

TABLE 1 Ex. 1 Ex. 2 fibers number fibers number Shaft prepreg angle of layers prepreg angle of layers A 8255S-10 0 deg. 4 3255G-10 0 deg. 4 B 9255S-10 45 deg.  4 9255S-10 45 deg.  4 C 9255S-10 −45 deg.    4 9255S-10 −45 deg.    4 D 9255S-10 0 deg. 1 9255S-10 0 deg. 1 E 8255S-10 0 deg. 1 9255S-10 0 deg. 1 F 3255G-10 0 deg. 1 3255G-10 0 deg. 1 G 8255S-10 0 deg. 2 8255S-10 0 deg. 2 Per-39 inch weight W (g) 30 30 Average bending rigidity Ela (kgf sq.m) 3.5 4.0 Lower limit of Ela by Exp. (1) (kgf sq.m) 3.5 3.5 Lower limit of Ela by Exp. (2) (kgf sq.m) 4.0 4.0 Lower limit of Ela by Exp. (3) (kgf sq.m) 4.0 4.0 Head speed 100 107 Launching angle 100 105 Directionality 100 88 Easiness of swing 4.7 4.9 Ex. 3 Ex. 4 fibers number fibers number Shaft prepreg angle of layers prepreg angle of layers A 3255G-10 0 deg. 4 8255S-10 0 deg. 4 B 9255S-10 45 deg.  4 9255S-10 45 deg.  4 C 9255S-10 −45 deg.    4 9255S-10 −45 deg.    4 D 9255G-10 0 deg. 1 9255S-10 0 deg. 1 E 9255G-10 0 deg. 1 8255S-10 0 deg. 3 F 9255S-10 0 deg. 1 3255G-10 0 deg. 2 G 8255S-10 0 deg. 2 8255S-10 0 deg. 2 Per-39 inch weight W (g) 30 45 Average bending rigidity Ela (kgf sq.m) 5.0 5.0 Lower limit of Ela by Exp. (1) (kgf sq.m) 3.5 4.25 Lower limit of Ela by Exp. (2) (kgf sq.m) 4.0 4.75 Lower limit of Ela by Exp. (3) (kgf sq.m) 4.0 5.0 Head speed 98 99 Launching angle 99 98 Directionality 105 101 Easiness of swing 4.7 4.7 Ex. 5 Ex. 6 fibers number fibers number Shaft prepreg angle of layers prepreg angle of layers A 8255S-10 0 deg. 4 8255S-10 0 deg. 4 B 9255S-10 45 deg.  4 9255S-10 45 deg.  4 C 9255S-10 −45 deg.    4 9255S-10 −45 deg.    4 D 9255S-10 0 deg. 1 9255S-10 0 deg. 1 E 8255S-10 0 deg. 1 8255S-10 0 deg. 2 F 3255G-10 0 deg. 4 3255G-10 0 deg. 6 G 8255S-10 0 deg. 2 8255S-10 0 deg. 2 Per-39 inch weight W (g) 45 60 Average bending rigidity Ela (kgf sq.m) 4.25 5.0 Lower limit of Ela by Exp. (1) (kgf sq.m) 4.25 5.0 Lower limit of Ela by Exp. (2) (kgf sq.m) 4.75 5.5 Lower limit of Ela by Exp. (3) (kgf sq.m) 5.0 6.0 Head speed 106 98 Launching angle 106 100 Directionality 86 100 Easiness of swing 4.9 4.7 Ref. 1 Ref. 2 fibers number fibers number Shaft prepreg angle of layers prepreg angle of layers A 8255S-10 0 deg. 4 8255S-10 0 deg. 4 B 9255S-10 45 deg.  4 9255S-10 45 deg.  4 C 9255S-10 −45 deg.    4 9255S-10 −45 deg.    4 D 8255S-10 0 deg. 1 8255S-10 0 deg. 1 E 3255G-10 0 deg. 3 3255G-10 0 deg. 5 F E1026A- 0 deg. 2 E1026A- 0 deg. 3 09N 09N G 8255S-10 0 deg. 2 8255S-10 0 deg. 2 Per-39 inch weight W (g) 45 60 Average bending rigidity Ela (kgf sq.m) 2.5 3.5 Lower limit of Ela by Exp. (1) (kgf sq.m) 4.25 5.0 Lower limit of Ela by Exp. (2) (kgf sq.m) 4.75 5.5 Lower limit of Ela by Exp. (3) (kgf sq.m) 5.0 6.0 Head speed 96 94 Launching angle 88 84 Directionality 173 226 Easiness of swing 3.5 3.1

From the test results, it was confirmed that the head speed and carry distance can be increased and the directionality can be improved.

As described above, in the FRP golf club shaft according to the present invention, since the club shaft weight converted into 39-inch club shaft length is set in a relatively light weight range, it is possible for the golfers with less powerful swings to swing the club head through the ball.

Further, since the average bending rigidity EIa of the club shaft is set in a specific range, the club shaft has a large bending rigidity, therefore, even if the club head is swung through at a high speed, an excessive flexure is avoided, and as a result, the directionality of the hit ball can be improved. 

1. A club shaft for an iron-type golf club made of a fiber reinforced resin, and having a per-39 inch weight W of not less than 30 grams and not more than 60 grams, and an average bending rigidity EIa of not more than 5.0 kgf sq.m, wherein an expression (1) EIa>=0.05 W+2.0 is satisfied.
 2. The club shaft according to claim 1, wherein an expression (2) EIa>=0.05 W+2.5 is further satisfied.
 3. The club shaft according to claim 1, wherein an expression (3) EIa>=(1/15)W+2.0 is further satisfied.
 4. An iron-type golf club comprising the club shaft according to claim 1, and an iron-type golf club head attached to the tip end of the club shaft.
 5. An iron-type golf club comprising the club shaft according to claim 2, and an iron-type golf club head attached to the tip end of the club shaft.
 6. An iron-type golf club comprising the club shaft according claim 3, and tip end of the club shaft. 